The desire to produce highly isomerized alkanes for high-octane transportation fuels has led to the use of bifunctional catalysts containing dispersed transition metals on acidic supports for alkane hydrocracking, as reported by Sullivan, F. R., Meyer, J. A. ACS Symp. Ser. 1975, No. 20, 28. Bifunctional catalysts consisting of hydrogenation components such as sulfided Ni, Mo, or W promoted on acidic supports such as Al.sub.2 O.sub.3 or SiO.sub.2 --Al.sub.2 O.sub.3 have been used; they crack n-alkanes to a significant extent but result in little isomerization. Noble metals, such as Pt and Pd, are strong hydrogenation catalysts which balance the acidity of supports and were reported to result in high selectivities to isomerized products in alkane hydrocracking, all as reported by W. A. van Hook and P. H. Emmett, J. Am. Chem. Soc. 1962, 14, 4410 and H. L. Conradt and W. E. Garwood, Ind. Eng. Chem. 1960, 52, 113. It was previously indicated that the amounts of isomerized and normal alkanes formed in n-alkanes hydrocracking depend on the relative strengths of the metal and acidic functions. Over the past two decades, a variety of Pt-promoted zeolites have been used as n-alkanes hydrocracking catalysts and have shown good selectivity to isokanes and increased resistance to heteroatom impurities.
Strong solid acid catalysts have high activity in cracking and skeletal isomerization of n-alkanes, as reported by Y. Nukano et al., J. Catal. 1979, 57, 1, J. C. Yori et al., Catal. Today 1989, 5, 493 and T. Hosoi et al., Am. Chem. Soc. Div. Petr. Chem. 1988, 562, and could serve as effective hydrocracking catalysts, especially when promoted by strong hydrogenation metals such as Pt and Ni. The metal-promoted anion-modified zirconium oxides (AZOs) such as Pt/ZrO.sub.2 /SO.sub.4 and Pt/ZrO.sub.2 /WO.sub.3, are strong solid acids with an active hydrogenation function; they could potentially replace the conventional bifunctional catalysts. There is increased interest in these catalysts because they have high activity at relatively low temperatures(&lt;200.degree. C.) favoring isomerization, are environmentally acceptable, are noncorrosive, are easily separated from process streams, and may be regenerated by calcination in air.
There has been much work reported on the use of these catalysts, especially sulfated zirconium oxide (SZO), for the isomerization of n-butane to isobutane, see T. Hosoi et al. Am. Chem. Soc. Div. Petr. Chem. 1988, 562, K. Tanabe et al., Crit. Rev. Surf. Chem. 1990, 1(1), 1, C. Morterra et al. Catal. 1994, 149, 181, F. Garin et al., J. Catal, 1991, 131, 199, P. Moles, Specialty Chemicals, Nov./Dec. 1992, M. A. Coehlo et al. Catal. Lett. 1995, 32, 253 and M. Hino et al., Catal. Lett. 1995, 30, 25, a reaction not catalyzed by 100% H.sub.2 SO.sub.4. Since superacids have been defined as acids stronger than 100% H.sub.2 SO.sub.4, the AZOs were considered to be solid superacids, but this classification is presently unsettled. Whether the activity and nature of acidity of these catalysts are attributable to Lewis or Bronsted acid sites or most likely to both is also under debate. Nevertheless, there is little doubt that carbocations are involved in most reactions catalyzed by AZOs. Pt-promoted SZO is reportedly a more active catalyst for higher alkane cracking and isomerization than most zeolite-based catalysts, see B. H. Davis et al., Catal. Today 1994, 20, 219.
Since there is a move to decrease aromatics and olefins in gasoline, the virtual absence of these compounds in the products may be an advantage of AZOs over zeolites. The cracked products obtained using SZOs also have more highly branched isoalkanes than those obtained using zeolites, since lower temperatures favor isomerization. M. Y. Wen et al. Energy & Fuels 1990, 4, 372, showed that Pt/ZrO.sub.2 /SO.sub.4 and Pt-promoted sulfated mixed oxides of zirconium and hafnium were active for the hydrocracking and hydroisomerization of n-heptane and n-hexadecane at 160.degree. C. and 360 psig (cold) H.sub.2. The Pt-SZO catalysts were also found to be active for reactions such as hydrocracking of the alkyl chain in long-chain alkylaromatics and for the alkylation of aromatics with alkanes.